Synthetic polyester yarns have been known and used commercially for several decades, having been first suggested by W. H. Carothers, U.S. Pat. No. 2,071,251, and then by Whinfield and Dickson, U.S. Pat. No. 2,465,319. Most such yarn is prepared in two stages, first by spinning (extruding) molten polymer to form undrawn filaments which are then drawn in a separate stage or separate process.
High strength polyester yarns are also well known, e.g., from Chantry and Molini, U.S. Pat. No. 3,216,187, and have been manufactured on a large scale and used commercially for more than 20 years. These commercial high strength yarns are often referred to as industrial yarns in contrast to apparel yarns. They have been characterized by their high tenacity (straight and loop). But I believe that industrial yarns that have excellent durability, as shown, e.g., by a good ability to withstand flexing, i.e. a good flex life, are preferred for for various industrial fabrics, e.g. tire cord, V-belts, sailcloth, automotive fabrics, and also for sewing thread. Many existing high strength (industrial) polyester yarns are of poly(ethylene terephthalate) of very high relative viscosity (measured herein as described hereinafter and sometimes referred to as LRV) about 38, corresponding to an intrinsic viscosity of about 0.9, and by a tenacity at break that is preferably about 10 g/d or more. There are also high strength industrial yarns of relative viscosity about 24, corresponding to an intrinsic viscosity of about 0.7, and by a tenacity at break of at least about 8 g/d. These higher viscosities of at least about 0.7 have distinguished these durable high strength industrial yarns from polyester apparel fabric yarns and from lower strength industrial yarns of lower viscosity, generally of relative viscosity up to about 21, corresponding to an intrinsic viscosity up to about 0.65, which may be regarded as regular viscosity for most textile purposes. Higher viscosities have been regarded as disadvantageous for most textile purposes. For many apparel purposes, the strength properties of even regular polyester have been a disadvantage, so that still lower viscosity polymer (e.g., 18) has been used, e.g., to reduce pilling in apparel. The present invention is not concerned with apparel yarns (from polymer of regular viscosity), but with high strength yarns only, from polymer of higher viscosity as disclosed, where resistance to flexing is believed by me to be of special advantage.
As disclosed, e.g., by Chantry and Molini, although it would have been expected that higher viscosity polyester would have given higher strength yarns, because of the higher molecular weight of the polymer, which otherwise (e.g., in nylon) could be expected to give higher strength yarns, until Chantry and Molini's invention it had not been possible to provide higher strength polyester yarns from polyester of higher viscosity. A high draw ratio has been considered an essential process element if high strength is desired. Until Chantry and Molini's invention it had not been possible to use high draw ratios with high viscosity polymer yarns. However, Chantry and Molini solved this problem by ensuring that the spinning conditions were such that there was an unusually low tension on the solidifying filaments, so that the spun yarn, before drawing, was characterized by an absence (i.e., a very low degree) of molecular orientation. This absence of orientation in this spun yarn, before drawing, was considered essential, otherwise the necessary high draw ratios were not achieved in the subsequent drawing operation. Accordingly, this low degree of orientation was believed to be essential for commercial production of high strength polyester yarns from high viscosity polymer, as disclosed, e.g., by Chantry and Molini, and as practiced commercially over the past two decades. It has been considered highly desirable to minimize aging of the undrawn yarn, and so it has been preferred to use a coupled process, in which the spinning and drawing stages are performed without intermediate wind-up, in order to develop such high strength yarns. Although a "split" process (in which undrawn yarn has been wound up first, and then drawn in a separate operation) has been and may still be practiced, it is recognized that the resulting drawn yarns are distinctly different from yarns prepared by a coupled process because of the different thermal histories. Much of the commercial research effort has, accordingly been devoted to aspects of coupled processes, for industrial polyester filament yarns, because of this prejudice against the split process.
In contrast to industrial polyester, for multifilament polyester apparel yarns (intrinsic viscosity up to about 0.65), however, during recent years by far the most popular process has been the preparation of textured polyester multifilament apparel yarns by a technique of first high speed melt-spinning polyester filaments to form a stable intermediate feed yarn that is partially-oriented (and, consequently, has been referred to by some people as POY for partially-oriented yarn), and then draw-texturing such intermediate feed yarn to produce the desired textured polyester yarn. It will be understood that the higher orientation in the intermediate POY is caused by higher tension in the solidifying filaments during the spinning process. This technique and the feed yarn were first disclosed by Petrille, U.S. Pat. No. 3,771,307, and Piazza and Reese, U.S. Pat. No. 3,772,872, and this process has been practiced commercially in many countries on a very large scale; in fact, for almost 20 years this technique has probably been the most widely-practiced technique worldwide in the whole synthetic polymer textile apparel industry (using polymer of intrinsic viscosity up to about 0.65).
However, so far as is known, few, if any, commercial high strength industrial polyester yarns have been made by high speed spinning of high viscosity polyester polymer to make a high viscosity partially-oriented intermediate yarn that is subsequently drawn to make the desired high strength industrial yarn. In this regard, a distinction should be made between (a) true high speed spinning to make a partially-oriented yarn, that is wound up as an undrawn yarn of low crystallinity, followed by a distinct separate drawing operation, in which crystallization occurs, and (b) processes as described, e.g., by Davis et al., U.S. Pat. Nos. 3,946,100 and 4,195,161, and by Yoshikawa, U.S. Pat. No. 3,997,175, who wind up at high speed a polyester yarn of low shrinkage (high crystallinity) using a step-wise process, involving first quenching the filaments, and then reheating these solidified filaments so that crystallization takes place before the yarn is wound, so as to form fully oriented polyester filaments, of low shrinkage before they are wound up for the first time.
There has always been a prejudice in favor of high deniers for industrial yarns. Many such yarns are typically of denier about 1,000 or more, and are plied together to form cords, which are generally effectively of lower tenacity than the constituent yarns or filaments in the final product. So there have been efforts towards increasing the denier per filament of industrial yarns, and interest in monofilaments, rather than in multifilament yarns.
It has been suggested by Hoechst, in German DE OS 3,431,831, published Mar. 13, 1986, that important changes occur in physical properties of polyester yarns after shrinking, with reduction in Tenacity (from 76 to 72 cN/tex) and with the development of a defect referred to as a "shrinkage saddle", and that high strength polyester yarns whose shrinkage at 200.degree. C. (S.sub.200) is as low as possible, and without any such "shrinkage saddle", can be produced by a hot-drawing process that is applied to highly preoriented filaments having a birefringence of at least 0.025 and an average molecular weight as defined by certain relative vicosity measurements; (no comparative tests are made and no discussion is given concerning use of starting materials that do not have the indicated molecular weight/viscosity, but controls are given to compare the effects of drawing less preoriented high viscosity materials). The hot-drawing must be carried out at a high draw ratio (90% of the maximum cold draw ratio) and within a narrow range of draw tensions that are low (19-23, preferably 20-23 cN/tex) whereby higher drawing temperatures are possible, indeed the temperatures used are so high that filaments with a low preorientation cannot be drawn safely. The drawing process is carried out on an assemblage of filaments, preferably using a belt path drawing device as shown in FIG. 3 of the publication, so it is impractical to define the temperature of drawing, this being determined by heat transfer and residence time, as well as the temperature of the device. The resulting filaments fall into two categories. Some are used directly as strength carriers, or as starting materials for twists (for tire cords), i.e., those that are not relaxed before such use (but usually receive another thermal treatment before being incorporated into a composite article); the Examples (10, 12, 4 and 8) show these (unrelaxed products) have S.sub.200 shrinkages of 5 to 6% with Tenacities of 68 to 72.5 cN/tex, in contrast to Controls (14, 5, 13, 1, 11 and 6) having S.sub.200 shrinkages of about 8 to almost 11%, and Tenacities from 69 through 83 cN/tex. For other uses, these unrelaxed shrinkages (5-6%) are too high, so the filaments are relaxed (after hot-drawing, on the same device) to give S.sub.200 shrinkages of 1.7 and 1.8% (Examples 7 and 9) with Tenacities of about 69 cN/tex, in contrast to relaxed Controls (3 and 2) having S.sub.200 shrinkages of 3.2 and 5.2% with Tenacities of 67.3 and 68.9 cN/tex. There is also limited discussion and less explanation of two newly-coined parameters referred to as SQ, stability quotient, and ED.sub.200 degree of elasticity, as well as of crystallinity limits, but all the unrelaxed Controls satisfy these requirements, which seem, therefore, to be an attempt only at distinguishing their drawn products from low shrinkage polyester yarns obtained by relaxation processes. This publication is directed at making high-titer (drawn) filaments for technical use. Fine filament titers are disparaged, as being more sensitive to chemicals. The total titer obtained by the various drawing processes in the Examples is always more than 1,100 dtex for the drawn filaments. The denier per filament is never expressly stated, but each spinneret had 100 holes, and 2 filaments were fed to a lubricating device together.